Load control device having a wide output range

ABSTRACT

A load control device (e.g., an LED driver) for controlling the intensity of a lighting load (e.g., an LED light source) may provide a wide output range and flicker-free adjustment of the intensity of the lighting load. The load control device may comprise a load regulation circuit, a control circuit, and a filter circuit (e.g., a boxcar filter circuit) that operates in a different manner in dependence upon a target current. When the intensity of the lighting load is near a low-end intensity, the control circuit may adjust an operating frequency of the load regulation circuit in response to the target current, and may control the filter circuit to filter a current feedback signal during a filter window that repeats on periodic basis. When the intensity of the lighting load is near a high-end intensity, the control circuit may control the filter circuit to constantly filter the current feedback signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/903,989, filed on Feb. 23, 2018, which claims the benefit of U.S.Provisional Patent Application No. 62/562,066, filed Sep. 22, 2017, andU.S. Provisional Patent Application No. 62/580,707, filed Nov. 2, 2017,the entire disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Light-emitting diode (LED) light sources (e.g., LED light engines) arereplacing conventional incandescent, fluorescent, and halogen lamps as aprimary form of lighting devices. LED light sources may comprise aplurality of light-emitting diodes mounted on a single structure andprovided in a suitable housing. LED light sources may be more efficientand provide longer operational lives as compared to incandescent,fluorescent, and halogen lamps. An LED driver control device (e.g., anLED driver) may be coupled between a power source, such as analternating-current (AC) power source or a direct-current (DC) powersource, and an LED light source for regulating the power supplied to theLED light source. For example, the LED driver may regulate the voltageprovided to the LED light source, the current supplied to the LED lightsource, or both the current and voltage.

Different control techniques may be employed to drive LED light sourcesincluding, for example, a current load control technique and a voltageload control technique. An LED light source driven by the current loadcontrol technique may be characterized by a rated current (e.g.,approximately 350 milliamps) to which the magnitude (e.g., peak oraverage magnitude) of the current through the LED light source may beregulated to ensure that the LED light source is illuminated to theappropriate intensity and/or color. An LED light source driven by thevoltage load control technique may be characterized by a rated voltage(e.g., approximately 15 volts) to which the voltage across the LED lightsource may be regulated to ensure proper operation of the LED lightsource. If an LED light source rated for the voltage load controltechnique includes multiple parallel strings of LEDs, a current balanceregulation element may be used to ensure that the parallel strings havethe same impedance so that the same current is drawn in each of theparallel strings.

The light output of an LED light source may be dimmed. Methods fordimming an LED light source may include, for example, a pulse-widthmodulation (PWM) technique and a constant current reduction (CCR)technique. In pulse-width modulation dimming, a pulsed signal with avarying duty cycle may be supplied to the LED light source. For example,if the LED light source is being controlled using a current load controltechnique, the peak current supplied to the LED light source may be keptconstant during an on-time of the duty cycle of the pulsed signal. Theduty cycle of the pulsed signal may be varied, however, to vary theaverage current supplied to the LED light source, thereby changing theintensity of the light output of the LED light source. As anotherexample, if the LED light source is being controlled using a voltageload control technique, the voltage supplied to the LED light source maybe kept constant during the on-time of the duty cycle of the pulsedsignal. The duty cycle of the load voltage may be varied, however, toadjust the intensity of the light output. Constant current reductiondimming may be used if an LED light source is being controlled using thecurrent load control technique. In constant current reduction dimming,current may be continuously provided to the LED light source. The DCmagnitude of the current provided to the LED light source, however, maybe varied to adjust the intensity of the light output.

Examples of LED drivers are described in U.S. Pat. No. 8,492,987, issuedJul. 23, 2013, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODELIGHT SOURCE; U.S. Pat. No. 9,655,177, issued May 16, 2017, entitledFORWARD CONVERTER HAVING A PRIMARY-SIDE CURRENT SENSE CIRCUIT; and U.S.Pat. No. 9,247,608, issued Jan. 26, 2016, entitled LOAD CONTROL DEVICEFOR A LIGHT-EMITTING DIODE LIGHT SOURCE; the entire disclosures of whichare hereby incorporated by reference.

SUMMARY OF THE INVENTION

As described herein, a load control device (e.g., an LED driver) forcontrolling the intensity of a lighting load (e.g., an LED light source)may provide a wide output range for current conducted through thelighting load to achieve flicker-free adjustment of the intensity of thelighting load. Since the load control device is characterized by a wideoutput range, the load control device may be able to control a largevariety of different lighting loads having different ratings (e.g.,different rated output currents and/or rated output voltages). This mayallow a manufacturer of the load control device, a manufacturer of afixture of the light source (such as an original equipment manufacturer(OEM)), and/or a distributer of the load control device and/or thefixture to maintain stock of a smaller number of stock keeping units(SKUs).

The load control device may comprise a load regulation circuit, acontrol circuit, and a filter circuit (e.g., such as a boxcar filtercircuit) that operates in a different manner in dependence upon theintensity (e.g., a target intensity) of the load control device in orderto provide the wide output range. The load regulation circuit maycontrol the magnitude of a load current conducted through the lightingload to adjust the intensity of the lighting load between a low-endintensity and a high-end intensity. The filter circuit may receive acurrent feedback signal from the load regulation circuit and filter thecurrent feedback signal to generate a filtered feedback signal. Thecontrol circuit may be operatively coupled to the load regulationcircuit for controlling the magnitude of the load current towards atarget current in response to the filtered feedback signal. When theintensity of the lighting load is near the low-end intensity (e.g., whenthe magnitude of the target current is less than a transition current),the control circuit may adjust an operating frequency of the loadregulation circuit in response to the target current, and may controlthe filter circuit to filter the current feedback signal during a filterwindow that repeats on a periodic basis. When the intensity of thelighting load is near the high-end intensity (e.g., when the magnitudeof the target current is greater than the transition current), thecontrol circuit may control the filter circuit to constantly filter thecurrent feedback signal. The control circuit may generate a filtercontrol signal for controlling the filter circuit to filter the currentfeedback signal during the filter window when the magnitude of thetarget current is less than the transition current, and control thefilter control signal to have a maximum duty cycle (e.g., 100%) when themagnitude of the target current is greater than the transition current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example light-emitting diode(LED) driver for controlling the intensity of an LED light source.

FIG. 2 is a simplified schematic diagram of an example LED driver.

FIG. 3A is an example plot of a relationship between an operatingfrequency and a target current of the LED driver of FIG. 2.

FIG. 3B is an example plot of a relationship between a magnitude of atarget-current control signal and the target current of the LED driverof FIG. 2.

FIGS. 4A and 4B show example waveforms illustrating the operation of theLED driver of FIG. 2.

FIG. 5A shows example waveforms illustrating the operation of the LEDdriver of FIG. 2 when the LED driver is learning a load voltage.

FIG. 5B shows example waveforms illustrating the operation of the LEDdriver of FIG. 2 when the LED driver is turning on an LED light sourceusing the learned load voltage.

FIG. 6 is a simplified flowchart of an example control procedure forcontrolling an LED driver to control a magnitude of a load currentconducted through a lighting load.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified block diagram of a load control device, such as alight-emitting diode (LED) driver 100 for controlling the intensity ofan LED light source 102 (e.g., an LED light engine). The LED lightsource 102 is shown in FIG. 1 as a plurality of LEDs connected in seriesbut may comprise a single LED or a plurality of LEDs connected inparallel or a suitable combination thereof, depending on the particularlighting system. In addition, the LED light source 102 may alternativelycomprise one or more organic light-emitting diodes (OLEDs). The LEDdriver 100 may be adapted to work with a plurality of different LEDlight sources, which may be rated at different magnitudes of loadcurrent and voltage.

The LED driver 100 may comprise a hot terminal H and a neutral terminalN for receiving an alternating-current (AC) voltage V_(AC) from an ACpower source (not shown). The LED driver 100 may comprise aradio-frequency (RFI) filter and rectifier circuit 110, which mayreceive the AC voltage V_(AC). The RFI filter and rectifier circuit 110may operate to minimize the noise provided on the AC power source and togenerate a rectified voltage V_(RECT). The LED driver 100 may comprise apower converter circuit 120, which may receive the rectified voltageV_(RECT) and generate a variable direct-current (DC) bus voltage V_(BUS)across a bus capacitor C_(BUS). The power converter circuit 120 maycomprise any suitable power converter circuit for generating anappropriate bus voltage, such as, for example, a boost converter, a buckconverter, a buck-boost converter, a flyback converter, a single-endedprimary-inductance converter (SEPIC), a Ćuk converter, or other suitablepower converter circuit. The power converter circuit 120 may alsoprovide electrical isolation between the AC power source and the LEDlight source 102, and operate as a power factor correction (PFC) circuitto adjust the power factor of the LED driver 100 towards a power factorof one.

The LED driver 100 may comprise a load regulation circuit, e.g., an LEDdrive circuit 130, which may receive the bus voltage V_(BUS) and controlthe amount of power delivered to the LED light source 102 so as tocontrol the intensity of the LED light source. For example, the LEDdrive circuit 130 may comprise a buck converter, as will be described ingreater detail below. To control the amount of power delivered to theLED light source 102, the LED drive circuit 130 may be configured tocontrol an average magnitude of a load current I_(LOAD) conductedthrough the LED light source 102.

The LED driver 100 may include a control circuit 140 for controlling theoperation of the power converter circuit 120 and the LED drive circuit130. The control circuit 140 may comprise, for example, a controller orany other suitable processing device, such as, for example, amicrocontroller, a programmable logic device (PLD), a microprocessor, anapplication specific integrated circuit (ASIC), or a field-programmablegate array (FPGA). The control circuit 140 may be configured to controlthe LED drive circuit 130 to control the average magnitude of the loadcurrent I_(LOAD) conducted through the LED light source to control theamount of power delivered to the LED light source. The control circuit140 may be configured to control the LED drive circuit 130 to turn theLED light source 102 on and off and to adjust (e.g., dim) a presentintensity L_(PRES) of the LED light source 102 towards a targetintensity L_(TRGT), which may range across a dimming range of the LEDlight source, e.g., between a low-end intensity L_(LE) (e.g.,approximately 0.1%-1.0%) and a high-end intensity L_(HE) (e.g.,approximately 100%).

The control circuit may be configured to fade (e.g., gradually adjustover a period of time) the target intensity L_(TRGT) (and thus thepresent intensity L_(PRES)) of the LED light source 102. The controlcircuit 140 may be configured to fade the LED light source 102 from offto on by slowly increasing the present intensity L_(PRES) of the LEDlight source from a minimum fading intensity L_(FADE-MIN), which may beless than the low-end intensity L_(LE) (e.g., such as approximately0.02%), to the target intensity L_(TRGT). The control circuit 140 may beconfigured to fade the LED light source 102 from on to off by slowlydecreasing the present intensity L_(PRES) of the LED light source froman initial intensity greater than or equal to the low-end intensityL_(LE) to the minimum fading intensity L_(FADE-MIN) at which point thecontrol circuit 140 may turn off the LED light source.

The control circuit 140 may be coupled to a memory 112 configured tostore operational characteristics of the LED driver 100 (e.g., thetarget intensity L_(TRGT), the low-end intensity L_(LE), the high-endintensity L_(HE), etc.). The memory 112 may be implemented as anexternal integrated circuit (IC) or as an internal circuit of thecontrol circuit 140. The LED driver 100 may also comprise acommunication circuit 114, which may be coupled to, for example, a wiredcommunication link or a wireless communication link, such as aradio-frequency (RF) communication link or an infrared (IR)communication link. The control circuit 140 may be configured todetermine the target intensity L_(TRGT) of the LED light source 102 orthe operational characteristics stored in the memory 112 in response todigital messages received via the communication circuit 114. In responseto receiving a command to turn on the LED light source 102, the controlcircuit 140 may be configured to execute the turn-on routine. The LEDdriver 100 may further comprise a power supply 116, which may receivethe rectified voltage V_(RECT) and generate a direct-current (DC) supplyvoltage V_(CC) (e.g., approximately 5 volts) for powering thelow-voltage circuitry of the LED driver. In addition, the power supply116 may generate one or more additional supply voltages, for example,for powering control circuitry of the power converter circuit 120 and/orthe LED drive circuit 130.

The control circuit 140 may comprise a digital control circuit, such asa processor 142, which may be, for example, a microprocessor, aprogrammable logic device (PLD), a microcontroller, an applicationspecific integrated circuit (ASIC), a field-programmable gate array(FPGA), or other suitable processing device or controller. The controlcircuit 140 may also comprise an analog control loop circuit 150. Theprocessor 142 and the analog control loop circuit 150 may operatetogether to control the LED driver circuit 130 to adjust the averagemagnitude of the load current I_(LOAD) towards a target currentI_(TRGT). The target current I_(TRGT) may be dependent upon the targetintensity L_(TRGT) (e.g., a function of the target intensity L_(TRGT)).The processor 142 may generate a target-current control signalV_(I-TRGT), which may have a DC magnitude or a duty cycle that mayindicate the target current I_(TRGT). The processor 142 may control theDC magnitude or the duty cycle of the target-current control signalV_(I-TRGT) based on the target intensity L_(TRGT) of the LED lightsource 102.

The control circuit 140 may also comprise a latch circuit 160 that maygenerate a drive signal V_(DR) for controlling the operation of the LEDdrive circuit 130 (e.g., for rendering a switching transistor of the LEDdrive circuit 130 conductive and non-conductive to regulate the averagemagnitude of the load current I_(LOAD) towards the target currentI_(TRGT)). The processor 142 may generate a frequency control signalV_(FREQ) that may set an operating frequency f_(OP) of the LED drivecircuit 130. In response to the frequency control signal V_(FREQ), thelatch circuit 160 may control the drive signal V_(DR) to render theswitching transistor of the LED drive circuit 130 conductive to start acycle of the LED drive circuit, at which time the LED drive circuit maybegin to conduct an inductor current I_(L) conducted through an inductor(not shown) of the LED drive circuit 130. The analog control loopcircuit 150 may generate a peak current threshold V_(TH-PK), which maybe used by the latch circuit 160 to render the switching transistor ofthe LED drive circuit 130 non-conductive in response to the magnitude ofthe inductor current I_(L).

The LED driver 100 may comprise an amplifier circuit 170, which mayreceive a current feedback signal V_(I-FB) from the LED drive circuit130. The amplifier circuit 170 may amplify the current feedback signalV_(I-FB) to generate an instantaneous current feedback signalV_(I-INST), which may indicate an instantaneous magnitude of theinductor current I_(L) flowing through the inductor of the LED drivecircuit 130.

The LED driver 100 may further comprise a filter circuit 180, such as aboxcar filter circuit. The filter circuit 180 may receive theinstantaneous current feedback signal V_(I-INST) and generate a filteredfeedback signal, e.g., an average current feedback signal V_(I-AVE),which may indicate an average magnitude of the inductor current I_(L)flowing through the inductor of the LED drive circuit 130 (e.g., over aspecific time window). The processor 142 may generate a filter controlsignal V_(FILTER) (e.g., a filter control signal) for controlling theoperation of the filter circuit 180, e.g., to control when the filtercircuit 180 filters the instantaneous current feedback signalV_(I-INST). For example, the processor 142 may control the filtercontrol signal V_(FILTER) to allow the filter circuit 180 to filter theinstantaneous current feedback signal V_(I-INST) over a filter windowperiod T_(FILTER) during each cycle of the LED drive circuit 130. Theprocessor 142 may control the filter control signal V_(FILTER) in amanner that is synchronous with the frequency control signal V_(FREQ),e.g., to start a cycle of the LED drive circuit 130 at the beginning ofthe filter window period T_(FILTER). For example, the filter windowperiod T_(FILTER) may have the same length during each cycle of the LEDdrive circuit 130 independent of the frequency of the frequency controlsignal V_(FREQ). The magnitude of the average current feedback signalV_(I-AVE) may indicate the average magnitude of the inductor currentI_(L) during the filter window period T_(FILTER) (e.g., while the filtercircuit 180 is filtering the instantaneous current feedback signalV_(I-INST)).

The analog control loop circuit 150 of the control circuit 140 mayreceive the average current feedback signal V_(I-AVE) and the latchcircuit 160 may receive the instantaneous current feedback signalV_(I-INST). The analog control loop circuit 150 may adjust the magnitudeof the peak current threshold V_(TH-PK) in response to thetarget-current control signal V_(I-TRGT) and the average currentfeedback signal V_(I-AVE). The latch circuit 160 may control the drivesignal V_(DR) to render the switching transistor of the LED drivecircuit 130 conductive in response to the frequency control signalV_(FREQ) (e.g., at the beginning of a cycle of the LED drive circuit130). The latch circuit 160 may control the drive signal V_(DR) torender the switching transistor non-conductive in response to the peakcurrent threshold V_(TH-PK) and the instantaneous current feedbacksignal V_(I-INST). After rendering the switching transistor of the LEDdrive circuit 130 non-conductive, the latch circuit 160 may remain in alatched state and maintain the switching transistor non-conductive untilthe beginning of the next cycle of the LED drive circuit 130.

The control circuit 140 may be configured to determine or learn (e.g.,measure or receive an indication of) one or more operationalcharacteristics of the LED light source 102 (e.g., learned loadcharacteristics). For example, the control circuit 140 may be configuredto determine a voltage representative of the magnitude of the loadvoltage V_(LOAD). The magnitude of the load voltage V_(LOAD) generatedacross the LED light source 102 may be dependent upon the magnitude ofthe load current I_(LOAD) (e.g., the target load current I_(TRGT) towhich the control circuit 140 is regulating the load current I_(LOAD))as well as the internal circuitry of the LED light source. The controlcircuit 140 may be configured to determine (e.g., measure) the magnitudeof the load voltage V_(LOAD) and/or store the measurement in the memory112 as a learned load voltage V_(LEARNED). The control circuit 140 maybe configured to determine (e.g., measure) the magnitude of the loadvoltage V_(LOAD) using a load voltage feedback signal V_(V-LOAD)received from the LED drive circuit 130. For example, the LED drivecircuit 130 may comprise a resistive divider circuit (not shown) coupledacross the LED light source 102 for generating the load voltage feedbacksignal V_(V-LOAD) as a scaled load voltage. The load voltage feedbacksignal V_(V-LOAD) may be received by an analog-to-digital converter(ADC) of the processor 142 for learning the magnitude of the loadvoltage V_(LOAD).

The control circuit 140 may be configured to determine (e.g., measure)the magnitude of the load voltage V_(LOAD) when the target intensityL_(TRGT) is at or near the low-end intensity L_(LE). For example, thecontrol circuit 140 may be configured to determine (e.g., measure) themagnitude of the load voltage V_(LOAD) while the control circuit 140 isfading the LED light source 102 from on to off, for example, while theaverage magnitude of the load current I_(LOAD) is within a measurementwindow that may range from a maximum learning threshold I_(LEARN-MAX) toa minimum learning threshold I_(LEARN-MIN). The maximum learningthreshold I_(LEARN-MAX) and the minimum learning threshold I_(LEARN-MIN)may be functions of a rated (or maximum) current I_(RATED) of the LEDlight source 102, for example, 0.0020·I_(RATED) and 0.0002·I_(RATED),respectively.

The control circuit 140 may be configured to control the LED drivecircuit 130 using the learned load voltage V_(LEARNED). For example, thecontrol circuit 140 may be configured to control the LED drive circuit130 in response to the learned load voltage V_(LEARNED) when turning onthe LED light source 102. The control circuit 140 may be configured tocharge (e.g., “pre-charge”) an output capacitor (not shown) of the LEDdrive circuit 130 prior to attempting to turn on the LED light source102. In response to receiving a command to turn on the LED light source102 and/or in response to power being applied to the LED driver 100 toturn on the LED light source, the control circuit 140 may pre-charge theoutput capacitor until the magnitude of the load voltage V_(LOAD)reaches or exceeds a pre-charge voltage threshold V_(TH-PC), which maybe, for example, a function of the learned load voltage V_(LEARNED)(e.g., as will be described in greater detail below). The pre-chargingof the output capacitor may allow the LED driver 100 to turn-on the LEDlight source 102 quickly and consistently, e.g., when fading on to thelow-end intensity L_(LE).

The control circuit 140 may be configured to determine an operatingparameter (e.g., a pre-load parameter) as a function of the learned loadvoltage V_(LEARNED) and use the operating parameter to control the LEDdrive circuit 130 to pre-charge the output capacitor of the LED drivecircuit 130 prior to turning the LED light source 102 on (e.g., as willbe described in greater detail below). For example, the control circuit140 may be configured to determine the DC magnitude or the duty cycle ofthe target-current control signal V_(I-TRGT) to use while pre-chargingthe output capacitor of the LED drive circuit 130 as a function of thelearned load voltage V_(LEARNED). In addition, the processor 142 maygenerate a start-up control signal V_(START-UP) for controlling theanalog control loop circuit 150 while pre-charging the output capacitorof the LED drive circuit 130 to maintain the output of the analogcontrol loop circuit 150 at a predetermined voltage.

After the magnitude of the load voltage V_(LOAD) reaches or exceeds thepre-charge voltage threshold V_(TH-PC), the processor 142 may controlthe start-up control signal V_(START-UP) to allow the analog controlloop circuit 150 to control the LED drive circuit 130 using closed loopcontrol in response to the current feedback signal V_(I-FB) to regulatethe magnitude of the load current LOAD towards the target currentI_(TRGT).

FIG. 2 is a simplified schematic diagram of a load regulation device,e.g., an LED driver 200 (such as the LED driver 100 of FIG. 1) forcontrolling the intensity of an LED light source 202. The LED driver 200may comprise a bus capacitor C_(BUS) for storing a bus voltage V_(BUS),which may be generated by a power converter circuit (e.g., the powerconverter circuit 120 of the LED driver 100). The LED driver 200 maycomprise an LED drive circuit 230, which may be configured to controlthe magnitude of a load current I_(LOAD) conducted through the LED lightsource 202. The LED driver 200 may further comprise a control circuit240, which may be a hybrid analog-digital control circuit (e.g., thecontrol circuit 140 of the LED driver 100). The control circuit 240 maycomprise a processor 242, a low-pass filter circuit 244, an analogcontrol loop circuit (e.g., which may include an integrator circuit250), and a latch circuit 260. The latch circuit 260 may generate adrive signal V_(DR), which may be provided to the LED driver circuit230. The LED driver 200 may further comprise an amplifier circuit 270and a filter circuit 280 (e.g., a boxcar filter circuit) for generatingan instantaneous current feedback signal V_(I-INST) and an averagecurrent feedback signal V_(I-AVE), respectively.

As shown in FIG. 2, the LED drive circuit 230 may comprise a buckconverter. The LED drive circuit 230 may comprise a switchingtransistor, e.g., a field-effect transistor (FET) Q232, which may becontrolled in response to the drive signal V_(DR) to control the averagemagnitude of the load current I_(LOAD). The LED drive circuit 230 mayalso comprise an inductor L234, a switching diode D235, an outputcapacitor C236, and a feedback resistor R238. The drive signal V_(DR)may be coupled to a gate of the FET Q232 through a gate drive circuit239. When the FET Q232 is conductive, the inductor L234 may conduct aninductor current I_(L) from the bus capacitor C_(BUS) through theparallel combination of the output capacitor C236 and the LED lightsource 202. When the FET Q232 is non-conductive, the inductor L234 mayconduct the inductor current I_(L) through the switching diode D235 andthe parallel combination of the output capacitor C236, and the LED lightsource 202. The LED light source 202 may conduct the average componentof the inductor current I_(L) and the output capacitor C236 may conductthe transient component of the inductor current I_(L). The averagemagnitude of the load current I_(LOAD) may be approximately equal to theaverage magnitude of the inductor current I_(L).

The current feedback signal V_(I-FB) may be generated across thefeedback resistor R238 of the LED drive circuit 230 and may beproportional to the magnitude of the inductor current I_(L). The currentfeedback signal V_(I-FB) may be received by the amplifier circuit 270.The amplifier circuit 270 may comprise an operational amplifier U272 andmay be configured as a non-inverting amplifier circuit. The operationalamplifier U272 may have a non-inverting input that may receive thecurrent feedback signal V_(I-FB). The amplifier circuit 270 may alsocomprise a resistor R274 coupled between an inverting input of theoperational amplifier U272 and circuit common, and a resistor R276coupled between the inverting input and an output of the operationalamplifier U272. The amplifier circuit 270 may be configured to generatethe instantaneous current feedback signal V_(I-INST), which may be anamplified version of the current feedback signal V_(I-FB) and mayindicate the instantaneous magnitude of the inductor current I_(L).

The filter circuit 280 may filter the instantaneous current feedbacksignal V_(I-INST) to generate the average load current signal V_(I-AVE),which may indicate the average magnitude of the inductor current I_(L).The filter circuit 280 may comprise a controllable switching circuit 282and a low-pass filter circuit (e.g., a third-order low-pass filtercircuit) that includes resistors R284, R286, R288 and capacitors C285,C287, C289. The processor 242 may generate a filter control signalV_(FILTER) for rendering the controllable switching circuit 282conductive and non-conductive. When the controllable switching circuit282 is conductive, the filter circuit 280 may be configured to filterthe instantaneous current feedback signal V_(I-INST) to generate theaverage current feedback signal V_(I-AVE). When the controllableswitching circuit 282 is non-conductive, the capacitors C285, C287, C289of the filter circuit 280 may maintain the magnitude of the averagecurrent feedback signal V_(I-AVE) at a value that indicates the averagemagnitude of the inductor current I_(L) during the period of time whenthe controllable switching circuit 282 was previously conductive.

The processor 242 may generate a pulse-width modulated (PWM) signalV_(PWM), which may be received by the low-pass filter circuit 244 of thecontrol circuit 240. The low-pass filter circuit 244 may be configuredto generate a target-current control signal V_(I-TRGT), which may have aDC magnitude that indicates the target current I_(TRGT). For example,the low-pass filter circuit 244 may comprise a resistor-capacitor (RC)circuit having a resistor R246 and a capacitor C248. The processor 242may be configured to control the duty cycle of the pulse-width modulatedsignal V_(PWM) to adjust the magnitude of the target-current controlsignal V_(I-TRGT).

The average current feedback signal V_(I-AVE) generated by the filtercircuit 280 and the target-current control signal V_(I-TRGT) generatedby the low-pass filter circuit 244 may be received by the integratorcircuit 250. The integrator circuit 250 may comprise an operationalamplifier U252 having a non-inverting input coupled to thetarget-current control signal V_(I-TRGT) and an inverting input coupledto the average current feedback signal V_(I-AVE) via a resistor R254.The integrator circuit 250 may comprise a capacitor C256 coupled betweenthe inverting input and an output of the operational amplifier U252,such that the integrator circuit 250 may be configured to integrate theerror between the average current feedback signal V_(I-AVE) and thetarget-current control signal V_(I-TRGT). The integrator circuit 250 maygenerate a peak current threshold V_(TH-PK) having a DC magnitude thatmay increase or decrease by amounts dependent upon the error between themagnitude of the target-current control signal V_(I-TRGT) and theaverage current feedback signal V_(I-AVE). The integrator circuit 250may comprise a controllable switching circuit 258 coupled in parallelwith the capacitor C256. The controllable switching circuit 258 may berendered conductive and non-conductive in response to a startup controlsignal V_(START-UP) received from the processor 242 during a startuproutine (e.g., as will be described in greater detail below).

The latch circuit 260 may receive the peak current threshold V_(TH-PK)generated by the integrator circuit 250 and the instantaneous currentfeedback signal V_(I-INST) generated by the amplifier circuit 270. Thelatch circuit 260 may comprise a comparator U262 configured to comparethe magnitude of the instantaneous current feedback signal V_(I-INST) tothe magnitude of the peak current threshold V_(TH). The comparator U262may generate a latch control signal V_(LATCH) at an output. When themagnitude of the instantaneous current feedback signal V_(I-INST) isless than the magnitude of the peak current threshold V_(TH), thecomparator U262 may drive the latch control signal V_(LATCH) at theoutput high (e.g., towards the supply voltage V_(CC)). When themagnitude of the instantaneous current feedback signal V_(I-INST)exceeds the magnitude of the peak current threshold V_(TH-PK), thecomparator U262 may drive the latch control signal V_(LATCH) at theoutput low (e.g., towards circuit common).

The processor 242 may generate a frequency control signal V_(FREQ) thatmay set an operating frequency f_(OP) of the LED drive circuit 230. Thelatch circuit 260 may comprise a PWM control circuit 266, which mayreceive the latch control signal V_(LATCH) from the comparator U262 andthe frequency control signal V_(FREQ) from the processor 242. The PWMcontrol circuit 266 may generate the drive signal V_(DR), which may bereceived by the gate drive circuit 239 of the LED drive circuit 230.When the frequency control signal V_(FREQ) is driven high at thebeginning of a cycle of the LED driver circuit 230, the PWM controlcircuit 266 may drive the magnitude of the drive signal V_(DR) high,which may render the FET Q232 of the LED drive circuit 230 conductive.When the magnitude of the instantaneous current feedback signalV_(I-INST) exceeds the magnitude of the peak current threshold signalV_(TH), the comparator U262 may drive the latch control signal V_(LATCH)low, which may cause the PWM control circuit 266 to drive the magnitudeof the drive signal V_(DR) low. The PWM control circuit 266 may maintainthe magnitude of the drive signal V_(DR) low until the processor 242drives the magnitude of the frequency control signal V_(FREQ) high onceagain at the end of the present cycle and the beginning of the nextcycle of the LED drive circuit 230.

The processor 242 may control the frequency of the frequency controlsignal V_(FREQ) and the duty cycle of the pulse-width modulated controlsignal V_(PWM) (and thus the magnitude of the target-current controlsignal V_(I-TRGT)) in dependence upon the target current I_(TRGT) of theLED light source 202 using open loop control. FIG. 3A is an example plotof the relationship 300 between the frequency of the frequency controlsignal V_(FREQ) (e.g., the operating frequency f_(OP) of the LED drivecircuit 230) and the target current I_(TRGT). FIG. 3B is an example plotof the relationship 310 between the magnitude of the target-currentcontrol signal V_(I-TRGT) and the target current I_(TRGT). For example,the target current I_(TRGT) may range between a high-end current I_(HE)(e.g., approximately 150 mA) at the high-end intensity L_(HE) and alow-end current I_(LE) (e.g., approximately 150 μA) at the low-endintensity L_(LE).

The processor 242 may operate in first and second modes of operationdepending upon whether the target current I_(TRGT) is less than orgreater than approximately a transition current I_(TRAN) (e.g.,approximately 16.8 mA). Near the low-end intensity L_(LE) (e.g., whenthe target current I_(TRGT) is less than approximately the transitioncurrent I_(TRAN)), the processor 242 may operate in the first operatingmode during which the processor 242 may adjust the frequency of thefrequency control signal V_(FREQ) between a minimum operating frequencyf_(MIN) and a maximum operating frequency f_(MAX) (e.g., linearly) withrespect to the target current I_(TRGT) while holding the magnitude ofthe target-current control signal V_(I-TRGT) constant (e.g., at aminimum voltage V_(MIN)). Near the high-end intensity L_(HE) (e.g., whenthe target current I_(TRGT) is greater than or equal to approximatelythe transition current I_(TRAN)), the processor 242 may operate in thesecond operating mode during which the processor 242 may adjust themagnitude of the target-current control signal V_(I-TRGT) between theminimum voltage V_(MIN) and a maximum voltage V_(MAX) (e.g., linearly)with respect to the target current I_(TRGT) while holding the frequencycontrol signal V_(FREQ) constant (e.g., at the maximum operatingfrequency f_(MAX)). For example, the maximum operating frequency f_(MAX)may be approximately 140 kHz and the minimum operating frequency f_(MIN)may be approximately 1250 Hz. For example, the maximum voltage V_(MAX)may be approximately 3.3 V and the minimum voltage V_(MIN) may beapproximately 44 mV.

FIGS. 4A and 4B show example waveforms illustrating the operation of theLED driver 200 shown in FIG. 2. FIG. 4A shows example waveformsillustrating the operation of the LED driver 200 when the target currentI_(TRGT) is less than the transition current I_(TRAN). The processor 242may generate the frequency control signal V_(FREQ) to set the operatingfrequency f_(OP) of the LED drive circuit 230. For example, an operatingperiod T_(OP) of the LED drive circuit 230 may be equal to the period ofthe frequency control signal V_(FREQ). The processor 242 may set theoperating frequency f_(OP) (and thus the operating period T_(OP)) independence upon the target current I_(TRGT) (e.g., as shown in FIG. 3A).The processor 242 may generate the frequency control signal V_(FREQ) tohave a predetermined on-time T_(FREQ-ON), which may have the same lengtheach cycle of the LED drive circuit 130 (e.g., independent of thefrequency of the frequency control signal V_(FREQ) or the target currentI_(TRGT)).

The processor 242 may generate the filter control signal V_(FILTER) in asynchronous manner with respect to the frequency control signalV_(FREQ). For example, the processor 242 may drive both the filtercontrol signal V_(FILTER) and the frequency control signal V_(FREQ) highat the same time to start a cycle of the LED drive circuit 230 (e.g., attime t₁ in FIG. 4A). At time t₁, the PWM control circuit 266 of thelatch circuit 260 may drive the magnitude of the drive signal V_(DR)high (e.g., towards the supply voltage V_(CC)) causing the FET Q232 ofthe LED drive circuit 230 to be rendered conductive. At this time, theinductor L234 of the LED drive circuit 230 may begin to conduct theinductor current I_(L). When the instantaneous current feedback signalV_(I-INST) (which may be proportional to the magnitude of the inductorcurrent I_(L)) exceeds the magnitude of the peak current thresholdsignal V_(TH), the PWM control circuit 266 may drive the magnitude ofthe drive voltage V_(DR) low (e.g., towards circuit common) as shown attime t₂ of FIG. 4A, which may cause the FET Q232 of the LED drivecircuit 230 to be rendered non-conductive. The drive signal V_(DR) maybe characterized by an on-time T_(ON) and a period that may be equal tothe operating period T_(OP) as shown in FIG. 4A. The PWM control circuit266 may render the FET Q232 conductive for the length of the on-timeT_(ON) of the drive signal V_(DR) during each operating cycle of the LEDdrive circuit 230. The inductor current I_(L) may have a peak magnitudeI_(PK) as shown in FIG. 4A. The magnitude of the inductor current I_(L)may begin to decrease at time t₂ until the magnitude of the inductorcurrent I_(L) drops to zero amps at time t₃.

The processor 242 may drive the frequency control signal V_(FREQ) low atthe end of the predetermined on-time T_(FREQ-ON) (e.g., at time t₄ inFIG. 4A). The processor 242 may drive the filter control signalV_(FILTER) low at the end of a filter window period T_(FILTER) (e.g., attime t₅ in FIG. 4A). The processor 242 may drive both the filter controlsignal V_(FILTER) and the frequency control signal V_(FREQ) high tostart another cycle of the LED drive circuit 230 at the end of theoperating period T_(OP) (e.g., at time t₆ in FIG. 4A).

When the target current I_(TRGT) is less than the transition currentI_(TRAN), the processor 242 may hold the magnitude of the target-currentcontrol signal V_(I-TRGT) constant at the minimum voltage V_(MIN), andlinearly adjust the frequency of the frequency control signal V_(FREQ)between the minimum frequency f_(MIN) and the maximum frequency f_(MAX)as a function of the target current I_(TRGT) (e.g., as shown in FIGS. 3Aand 3B). The filter circuit 280 may be configured to filter theinstantaneous current feedback signal V_(I-INST) during the filterwindow period T_(FILTER) each cycle of the LED drive circuit 230. Whenthe target current I_(TRGT) is less than the transition currentI_(TRAN), the filter control signal V_(FILTER) may be a periodic signalcharacterized by the operating frequency f_(OP). The processor 242 maymaintain the length of the filter window period T_(FILTER) of the filtercontrol signal V_(FILTER) constant from one cycle of the LED drivercircuit 230 to the next cycle independent of the frequency of thefrequency control signal V_(FREQ). A duty cycle of the filter controlsignal V_(FILTER) may vary as the frequency of the frequency controlsignal V_(FREQ) is adjusted.

Since the target-current control signal V_(I-TRGT) and the filter windowperiod T_(FILTER) are held constant, the on-time T_(ON) of the drivesignal V_(DR) may be approximately the same each cycle of the LED drivecircuit 230 even though the frequency of the drive signal V_(DR) (e.g.,the operating period T_(OP)) may vary in dependence upon the targetcurrent I_(TRGT). As a result, the peak and average magnitudes of theinductor current I_(L) during the filter window period T_(FILTER) may beapproximately the same from one cycle to the next of the LED drivecircuit 230 independent of the target current I_(TRGT) when the targetcurrent I_(TRGT) is less than the transition current I_(TRAN). Thelength of the filter window period T_(FILTER) may be sized to ensurethat the inductor current I_(L) drops to zero amps before the end of thefilter window period T_(FILTER) when the target current I_(TRGT) is lessthan the transition current I_(TRAN). When the target current is lessthan the transition current I_(TRAN), the LED drive circuit 230 may beconfigured to operate in a discontinuous mode of operation.

FIG. 4B shows example waveforms illustrating the operation of the LEDdriver 200 when the target current I_(TRGT) is greater than thetransition current I_(TRAN). When the target current I_(TRGT) is greaterthan the transition current I_(TRAN), the processor 242 may linearlyadjust the magnitude of the target-current control signal V_(I-TRGT)between the minimum voltage V_(MIN) and the maximum voltage V_(MAX) as afunction of the target current I_(TRGT) (e.g., as shown in FIGS. 3A and3B). In addition, the processor 242 may hold the frequency of thefrequency control signal V_(FREQ) constant at the maximum operatingfrequency f_(MAX) (e.g., causing the operating period T_(OP) to be heldconstant at a minimum operating period T_(MIN)). When the target currentI_(TRGT) is greater than the transition current I_(TRAN), the processor242 may control the duty cycle of the filter control signal V_(FILTER)to a maximum filter duty cycle (e.g., 100%). For example, the operatingperiod T_(OP) may be equal to the length of the filter window periodT_(FILTER) when the target current I_(TRGT) is greater than thetransition current I_(TRAN). As a result, the processor 242 may drivethe filter control signal V_(FILTER) high at all times (e.g., the filtercontrol signal V_(FILTER) is a constant signal) while the target currentI_(TRGT) is greater than the transition current I_(TRAN) as shown inFIG. 4B. The average current feedback signal V_(I-AVE) may indicate theaverage magnitude of the inductor current I_(L) when the target currentI_(TRGT) is greater than the transition current I_(TRAN). Additionallyor alternatively, the processor 242 may drive the filter control signalV_(FILTER) high approximately all of the time (e.g., almost all of thetime), for example at substantially large duty cycle (e.g.,approximately 90% or greater).

Because the processor 242 varies the magnitude of the target-currentcontrol signal V_(I-TRGT) as a function of the target current I_(TRGT),the length of the on-time T_(ON) of the drive signal V_(DR) may vary asa function of the target current I_(TRGT) even though the frequency ofthe drive signal V_(DR) (e.g., the operating period T_(OP)) is heldconstant. As the target current I_(TRGT) increases, the peak currentI_(PK) of the inductor current may increase to a point at which the LEDdrive circuit 230 may begin to operate in a continuous mode ofoperation. Since the minimum operating period T_(MIN) (e.g., theoperating period T_(OP) when the target current I_(TRGT) is greater thanthe transition current I_(TRAN)) may be equal to the length of thefilter window time period T_(FILTER), the processor 242 may beconfigured to smoothly transition the LED driver 200 between the firstoperating mode when the target current I_(TRGT) is less than thetransition current I_(TRAN) and the second operating mode when thetarget current I_(TRGT) is greater than the transition current I_(TRAN).

The length of the predetermined on-time T_(FREQ-ON) of the frequencycontrol signal T_(FREQ) is less than the length of the operating periodT_(OP) when the target current I_(TRGT) is greater than the transitioncurrent I_(TRAN). The processor 242 may drive the frequency controlsignal T_(FREQ) low (e.g., at time t₇ in FIG. 4B) and then high (e.g.,at time t₈) at the end of each cycle of the LED drive circuit 230. Thiscauses the PWM control circuit 266 of the latch circuit 260 to stopmaintaining the magnitude of the drive signal V_(DR) low, and to drivethe magnitude of the drive signal V_(DR) high again when the frequencycontrol signal T_(FREQ) is driven high to begin the next cycle of theLED drive circuit 230 (e.g., at time t₈).

The processor 242 of the control circuit 240 may be configured todetermine or learn (e.g., measure or receive an indication of) themagnitude of the load voltage V_(LOAD) and/or store the measurement inmemory (e.g., the memory 112) as a learned load voltage V_(LEARNED). Themagnitude of the load voltage V_(LOAD) generated across the LED lightsource 202 may be dependent upon the magnitude of the load currentI_(LOAD) (e.g., the target load current I_(TRGT) to which the controlcircuit 240 is regulating the load current I_(LOAD)) as well as theinternal circuitry of the LED light source. The processor 242 may beconfigured to receive a load voltage feedback signal from the LED drivecircuit 230 (e.g., the load voltage feedback signal V_(V-LOAD) of theLED driver 100), which may be a scaled version of the load voltageV_(LOAD) generated by a resistive divider circuit (not shown) of the LEDdrive circuit 230. The processor 242 may sample the load voltagefeedback signal using an analog-to-digital converter (ADC) to measurethe magnitude of the load voltage V_(LOAD).

FIG. 5A shows example waveforms illustrating the operation of the LEDdriver 200 when the processor 242 is learning the load voltage V_(LOAD).The processor 242 may be configured to determine (e.g., measure) themagnitude of the load voltage V_(LOAD) while the processor 242 is fadingthe LED light source 202 from on to off. As shown in FIG. 5A, whenfading the LED light source 202 from on to off, the processor 242 maybegin to decrease the average magnitude of the load current I_(LOAD)from an initial current I_(INIT) at time t₀, at which time the magnitudeof the load voltage V_(LOAD) may also begin to decrease, e.g., from aninitial voltage V_(INIT). The processor 242 may be configured todetermine (e.g., measure) the magnitude of the load voltage V_(LOAD)while the average magnitude of the load current I_(LOAD) is within ameasurement window that may range from a maximum learning thresholdI_(LEARN-MAX) and a minimum learning threshold I_(LEARN-MIN) (e.g.,between times t_(WIN-START) and t_(WIN-END) as shown in FIG. 5A). Themaximum learning threshold I_(LEARN-MAX) and the minimum learningthreshold I_(LEARN-MIN) may be functions of a rated (or maximum) currentI_(RATED) of the LED light source 202, for example, 0.0020·I_(RATED) and0.0002·I_(RATED), respectively. The processor 242 may be configured toperiodically sample the load voltage feedback signal during themeasurement window, and to process the plurality of samples to determinethe learned load voltage V_(LEARNED). For example, the processor 242 maybe configured to process the plurality of samples of the load voltagefeedback signal by calculating an average or median value of theplurality of samples or filtering the samples using a digital low-passfilter.

The processor 242 may be configured to measure the load voltage V_(LOAD)and determine the learned load voltage V_(LEARNED) when (e.g., each timethat) the processor 242 turns the LED light source 202 off (e.g., fadesthe LED light source off). The processor 242 may be configured tooverwrite the learned load voltage V_(LEARNED) stored in the memory withthe learned load voltage V_(LEARNED) determined the last time that theprocessor 242 turned off the LED light source 202. In addition, theprocessor 242 may be configured to process the learned load voltagesV_(LEARNED) from multiple turn-off events (e.g., calculate the averageor median value of the multiple learned load voltages) beforeoverwriting the learned load voltage V_(LEARNED) stored in the memory.

The processor 242 may be configured to control the LED drive circuit 230using the learned load voltage V_(LEARNED), for example, when turning onthe LED light source 202. FIG. 5B shows example waveforms illustratingthe operation of the LED driver 200 when the processor 242 is fading onthe LED light source 202 (e.g., fading on to a target intensity L_(TRGT)that corresponds to a target current I_(TRGT)). In response to receivinga command to turn on the LED light source 202 and/or in response topower being applied to the LED driver 200 to turn on the LED lightsource, the processor 242 may be configured to pre-charge the outputcapacitor C236 of the LED drive circuit 230 during a pre-charge periodT_(PRE-CHARGE) prior to attempting to turn on the LED light source 202.During the pre-charge period T_(PRE-CHARGE), the processor 242 may beconfigured to control the duty cycle of the pulse-width modulated signalV_(PWM) (and thus the DC magnitude of the target-current control signalV_(I-TRGT)) as a function of the learned load voltage V_(LEARNED) tocause the output capacitor C236 to charge faster than normal (e.g.,faster than if the processor 242 controlled the DC magnitude of thetarget-current control signal V_(I-TRGT) in response to the targetcurrent I_(TRGT) as shown in FIG. 3B). The faster rate at which theoutput capacitor C236 charges during the pre-charge periodT_(PRE-CHARGE) may allow the processor 242 to turn-on the LED lightsource 202 quickly and consistently, e.g., when fading the LED lightsource on to the low-end intensity L_(LE).

The control circuit 240 may be configured to pre-charge the outputcapacitor C236 of the LED drive circuit 230 until the magnitude of theload voltage V_(LOAD) reaches or exceeds a pre-charge voltage thresholdV_(TH-PC). The pre-charge voltage threshold V_(TH-PC) may be determined,for example, as a function of the learned load voltage V_(LEARNED)(e.g., V_(TH-PC)=α·V_(LEARNED), where α is a constant that may be, forexample, approximately 0.90). Since the magnitude of the load voltageV_(LOAD) may be greater when the LED light source 202 is cold than whenthe LED light source 202 is warm, the constant α may be sized to be lessthan one to ensure that the LED drive circuit 230 does not overshoot thelearned load voltage V_(LEARNED) when pre-charging the output capacitorC236. Additionally or alternatively, the pre-charge voltage thresholdV_(TH-PC) may be determined, for example, using a different function ofthe learned load voltage V_(LEARNED) (e.g., V_(TH-PC)=V_(LEARNED)−β,where β is a constant that may be, for example, approximately one volt).Additionally or alternatively, the pre-charge voltage thresholdV_(TH-PC) may be a fixed threshold (e.g., a predetermined threshold).The processor 242 may be configured to cease pre-charging the outputcapacitor C236 if the magnitude of the load voltage V_(LOAD) does notexceed the pre-charge voltage threshold V_(TH-PC) within a timeoutperiod. The processor 242 may be configured to select the value of theduty cycle of the pulse-width modulated signal V_(PWM) based on thelearned load voltage V_(LEARNED) such that pre-charge periodT_(PRE-CHARGE) for the LED driver 200 may be approximately the same fordifferent LED light sources that have different resulting load voltages.

The processor 242 may control the start-up control signal V_(START-UP)to render the controllable switching circuit 258 of the integratorcircuit 250 conductive during the pre-charge period T_(PRE-CHARGE).After the magnitude of the load voltage V_(LOAD) reaches or exceeds thepre-charge voltage threshold V_(TH-PC), the processor 242 may controlthe start-up control signal V_(START-UP) to render the controllableswitching circuit 258 of the integrator circuit 250 non-conductive. Thismay allow the integrator circuit 250 and the latch circuit 260 tocontrol the LED drive circuit 230 using closed loop control in responseto the current feedback signal V_(I-FB) to regulate the magnitude of theload current I_(LOAD) towards the target current I_(TRGT).

FIG. 6 is a simplified flowchart of an example control procedure 600 forcontrolling a load control device (e.g., the LED driver 200) to controla magnitude of a load current conducted through a lighting load (e.g.,the LED light source 202). The control procedure 600 may be executed bya control circuit of the load control device (e.g., the control circuit240 of the LED driver 200) at step 610, for example, periodically and/orin response to a change in the target current I_(TRGT) for the lightingload. If the target current I_(TRGT) is less than the transition currentI_(TRAN) at 612 (e.g., when the target intensity L_(TRGT) in near thelow-end intensity L_(LE)), the control circuit may maintain themagnitude of the target-current control signal V_(I-TRGT) constant(e.g., at the minimum voltage V_(MIN)) at 614, and may adjust thefrequency of the frequency control signal V_(FREQ) in response to thetarget current I_(TRGT) (e.g., as shown in FIG. 3A) at 616. The controlcircuit may then control a filter circuit (e.g., the filter circuit 280)at 618 by controlling the filter control signal V_(FILTER) to beperiodic (e.g., having the same frequency of the frequency controlsignal V_(FREQ)) and synchronized to the frequency control signalV_(FREQ) (e.g., as shown in FIG. 4A). The control procedure 600 may thenexit.

If the target current I_(TRGT) is greater than the transition currentI_(TRAN) (e.g., greater than or equal to the transition currentI_(TRAN)) at 612 (e.g., when the target intensity L_(TRGT) in near thehigh-end intensity L_(HE)), the control circuit may maintain thefrequency of the frequency control signal V_(FREQ) constant (e.g., atthe maximum operating frequency f_(MAX)) at 620, and may adjust themagnitude of the target-current control signal V_(I-TRGT) in response tothe target current I_(TRGT) (e.g., as shown in FIG. 3B) at 622. Thecontrol circuit may then control the filter control signal V_(FILTER) tobe substantially constant at 624, before the control procedure 600exits. For example, the control circuit may drive the control signalV_(FILTER) using a maximum duty cycle, such as 100% (e.g., by constantlydriving the filter control signal V_(FILTER) high as shown in FIG. 4B),or a substantially high duty cycle (e.g., 90% or greater) at 624.

What is claimed is:
 1. A light-emitting diode (LED) driver forcontrolling an intensity of an LED light source, the LED drivercomprising: an LED drive circuit configured to control the magnitude ofa load current conducted through the LED light source to adjust theintensity of the LED light source between a low-end intensity and ahigh-end intensity, the LED drive circuit characterized by an operatingfrequency; a filter circuit configured to receive a current feedbacksignal from the LED drive circuit and to filter the current feedbacksignal to generate a filtered feedback signal; and a control circuitoperatively coupled to the LED drive circuit for controlling themagnitude of the load current towards a target current in response tothe filtered feedback signal; wherein the control circuit is configuredto adjust the operating frequency of the LED drive circuit in responseto the target current when the intensity of the LED light source is nearthe low-end intensity, the control circuit configured to control thefilter circuit to filter the current feedback signal during a filterwindow that repeats on a periodic basis when the intensity of the LEDlight source is near the low-end intensity, the control circuitconfigured to control the filter circuit to constantly filter thecurrent feedback signal when the intensity of the LED light source isnear the high-end intensity.
 2. The LED driver of claim 1, wherein thecontrol circuit is configured to adjust the operating frequency of theLED drive circuit in response to the target current when the magnitudeof the target current is less than a transition current, the controlcircuit configured to control the filter circuit to filter the currentfeedback signal during the filter window when the magnitude of thetarget current is less than the transition current, the control circuitconfigured to control the filter circuit to constantly filter thecurrent feedback signal when the magnitude of the target current isgreater than the transition current.
 3. The LED driver of claim 2,wherein the LED drive circuit comprises a semiconductor switch and thecontrol circuit is configured to render the semiconductor switchconductive for an on-time during each operating cycle of the LED drivecircuit.
 4. The LED driver of claim 3, wherein, when the magnitude ofthe target current is greater than the transition current, the controlcircuit is configured to maintain the operating frequency of the LEDdrive circuit constant and adjust the on-time of the semiconductorswitch of the LED drive circuit in response to the target current. 5.The LED driver of claim 4, wherein the control circuit is configured toadjust a magnitude of a target-current control signal to adjust theon-time of the semiconductor switch of the LED drive circuit when themagnitude of the target current is greater than the transition current.6. The LED driver of claim 5, wherein, when the magnitude of the targetcurrent is less than the transition current, the control circuit isconfigured to maintain the magnitude of the target-current controlsignal constant and adjust the operating frequency of the LED drivecircuit in response to the target current.
 7. The LED driver of claim 2,wherein the control circuit is configured to control the filter circuitwith a periodic signal when the magnitude of the target current is lessthan the transition current, and with a constant signal when themagnitude of the target current is greater than the transition current.8. The LED driver of claim 7, wherein the control circuit is configuredto generate a frequency control signal for controlling the operatingfrequency of the LED drive circuit.
 9. The LED driver of claim 8,wherein the periodic signal for controlling the filter circuit has aconstant on-time.
 10. The LED driver of claim 9, wherein the periodicsignal for controlling the filter circuit is synchronized with thefrequency control signal.
 11. The LED driver of claim 10, wherein theon-time of the periodic signal for controlling the filter circuit isequal to a minimum period of the frequency control signal.
 12. The LEDdriver of claim 7, wherein the filter circuit comprises a boxcar filtercircuit.
 13. The LED driver of claim 4, wherein the control circuitcomprises a digital control circuit configured to generate atarget-current control signal and an analog control loop circuitconfigured to control the semiconductor switch of the LED drive circuitto control the magnitude of the load current in response to thetarget-current control signal.
 14. The LED driver of claim 13, wherein,when the magnitude of the target current is less than the transitioncurrent, the control circuit is configured to maintain the magnitude ofthe target-current control signal constant and adjust the operatingfrequency of the LED drive circuit in response to the target current.15. A light-emitting diode (LED) driver for controlling an intensity ofan LED light source, the LED driver comprising: a LED drive circuitconfigured to control the magnitude of a load current conducted throughthe LED light source to adjust the intensity of the LED light source; afilter circuit configured to receive a current feedback signal from theLED drive circuit and to filter the current feedback signal to generatea filtered feedback signal; and a control circuit operatively coupled tothe LED drive circuit for controlling the magnitude of the load currenttowards a target current in response to the filtered feedback signal;wherein the control circuit is configured to adjust an operatingfrequency of the LED drive circuit in response to the target currentwhen the magnitude of the target current of the LED light source is lessthan a transition current, the control circuit configured to control thefilter circuit to filter the current feedback signal during a filterwindow that repeats on a periodic basis when the magnitude of the targetcurrent of the LED light source is less than the transition current, thecontrol circuit configured to control the filter circuit to constantlyfilter the current feedback signal when the magnitude of the targetcurrent of the LED light source is greater than the transition current.16. The LED driver of claim 15, wherein the control circuit isconfigured to: render a semiconductor switch of the LED drive circuitconductive for an on-time during each operating cycle of the LED drivecircuit; when the magnitude of the target current is greater than thetransition current, maintain the operating frequency of the LED drivecircuit constant and adjust the on-time of the semiconductor switch ofthe LED drive circuit in response to the target current; and when themagnitude of the target current is less than the transition current,adjust the operating frequency of the LED drive circuit in response tothe target current.
 17. The LED driver of claim 15, wherein the controlcircuit is configured to control the filter circuit with a periodicsignal when the magnitude of the target current is less than thetransition current, and with a constant signal when the magnitude of thetarget current is greater than the transition current.
 18. Alight-emitting diode (LED) driver for controlling an intensity of an LEDlight source, the LED driver comprising: a LED drive circuit configuredto control the magnitude of a load current conducted through the LEDlight source to adjust the intensity of the LED light source between alow-end intensity and a high-end intensity; a filter circuit configuredto receive a current feedback signal from the LED drive circuit and tofilter the current feedback signal to generate a filtered feedbacksignal; and a control circuit operatively coupled to the LED drivecircuit for controlling the magnitude of the load current towards atarget current in response to the filtered feedback signal; wherein thecontrol circuit is configured to adjust an operating frequency of theLED drive circuit in response to the target current when the magnitudeof the target current is less than a transition current, the controlcircuit configured to generate a filter control signal for controllingthe filter circuit to filter the current feedback signal during a filterwindow that repeats on a periodic basis when the magnitude of the targetcurrent is less than the transition current, the control circuitconfigured to control the filter control signal to have a maximum dutycycle when the magnitude of the target current is greater than thetransition current.
 19. The LED driver of claim 18, wherein the filtercontrol signal is a periodic signal when the target current is less thanthe transition current and a constant signal when the target current isgreater than the transition current.
 20. The LED driver of claim 19,wherein the control circuit is configured to generate a frequencycontrol signal for controlling the operating frequency of the LED drivecircuit, the control circuit configured to control the periodic signalto have a constant on-time and to be synchronized with the frequencycontrol signal.
 21. The LED driver of claim 20, wherein the maximum dutycycle is 100%.
 22. The LED driver of claim 20, wherein the maximum dutycycle is greater than approximately 90%.